Radiation conversion is usually carried out because of non-linear interaction which frequently comes about because the electric field strength of the field of light irradiating a crystal is no longer negligible in comparison with the nuclear (electric) field of the material exposed to the radiation. Since the discovery of the laser in 1960, the high electric field intensities which can be achieved with these coherently radiating light sources have consequently resulted in the observation of numerous new kinds of interaction between light and material (see, for example, N. Bloembergen: Nonlinear Optics, W. A. Benjamin, New York 1965).
Crystals, which, owing to their particular non-linearly optical properties, are used for the frequency conversion, in general, of laser radiation in the 250-3000 nm wavelength range, originate from very diverse substance families, each of which has its own very specific properties. The most important types of crystal currently used are KTiOPO.sub.4 (KTP), .beta.-BaB.sub.2 O.sub.4 (BBO), LiB.sub.3 O.sub.5 (LBO), LiIO.sub.3 and the newer compounds CsLiB.sub.6 O.sub.10 (CLBO) and Ca.sub.4 GdO(BO.sub.3).sub.3. Since non-linear optical processes are not only dependent upon the specific non-linear optical properties, but also upon the linear optical properties (e.g. dispersion of the powers and absorption), and, decisively, also upon the growth capacity and workability of the crystals, the types of crystals already used have various drawbacks depending upon their application.
The commercially used crystals KTB, BBO and LBO all have the drawback that they have to be grown from melt solvents. With this method, crystal growth is very slow, and several weeks or even months are needed to obtain crystals which are of a sufficient size for most applications. Moreover, with this method of growth, only a small yield of crystal volume is obtained per volume of molten mixture used. The optical quality of crystals from melt solutions is mostly inadequate because these crystals usually have micro-inclusions of melt solvents.
On the other hand, crystals with non-linearly optical properties which are grown according to the Czochralski method, the Bridgman method, or the top-seeding method, are mostly better in terms of optical quality.
Another advantage of these methods is that in a relatively short space of time far larger crystals can be grown. Examples of such crystals are CLBO, Ca.sub.4 GdO(BO.sub.3).sub.3 or Ca.sub.4 YO(BO.sub.3).sub.3. However, these crystals have a smaller frequency conversion efficiency because they have smaller non-linearly optical coefficients d.sub.ijk than the substances KTP, BBO and LBO.
The non-linear optical coefficients are shown in Table 1.
TABLE 1 KTP.sup.1) BBO.sup.1) LBO.sup.1) CLBO.sup.1) Ca.sub.4 GdO(BO.sub.3).sub.3.sup.2) d.sub.311 = 1.4 d.sub.311 = .+-.0.16 d.sub.311 = +0.67 d.sub.123 = d.sub.122 = 0.56 0.95 d.sub.322 = 2.65 d.sub.222 = +2.3 d.sub.322 = +0.85 d.sub.322 = 0.44 (d.sub.333 = 10.7) d.sub.333 = .+-.0.04 .sup.1) V. G. Dmitriev et al.: Handbook of Nonlinear Optical Crystals, Springer Verlag, 2nd Edition (1997) .sup.2) Aka et al.: World Patent No. WO 96/26464